Controversies: Genetic and environmental influences
Authored by Henry Strick van Linschoten
All biologists recognise that under no circumstances can genes and the environment function or exist without each other: all life consists of cells and all cells contain DNA. The DNA molecule itself is inert and cannot be connected with any activity or influence unless it is embedded in a living cell, while a cell cannot live on its own, but needs an environment in which to live. Genetic influences are traditionally contrasted with environmental effects. Biologists define environment in to cover everything that is not inherited from the DNA of the parents. “Environment” here includes the following ():
- the biophysical environment surrounding the organism, including its chemical and physical aspects / components, both living and non-living and that may interact with the organism by the exchange of mass, energy or information
- developmental influences such as parenting, family, local community, education, groups, culture
- prenatal events, between conception and birth, including the environment over that period
- pre- and post-birth events such as illness, nutrition, toxicity, environmental load through noise, pollution, radiation
- changes in the DNA that are not inherited because they occur in cells other than testes and ovaries (including environmentally induced changes in DNA)
- when taking a systems view, its components and its interactions seen as one whole environmental system
The consequences of this wider view of environment are considerable. In particular, the interchangeable use of the terms innate and inherited is questionnable. What is innate at birth (e.g., instincts, functioning senses, reflexes, certain behavioural systems) is not present when the first cell of the organism was conceived. It must therefore have come about during embryological development. Hence it is just as likely to have been influenced by the environmental period in the womb as have been “programmed” genetically.
More terminology is needed to describe the factors behind a completed. Biologists define epigenetic as referring to the interaction between inherited DNA-genetic factors and developmental processes, of whatever type, through which the genotype is expressed in the phenotype. Epigenetic factors and processes are again under the influence of both the available genes and the environment (as it changes over time).
Any expressed phenotype of an organism (or one of its traits) is always the result of a combination of genes and environment from conception until the time that the phenotype is under observation. There are several groups of factors that can influence the gene expression and the development (ontogeny) of a particular phenotype. A gene is being expressed by a stretch of DNA being copied into an mRNA molecule (messenger RNA), which is then used in another process for the synthesis of a protein. Epigenetic factors are a generic name for all the different processes over time that can influence how the genes are used to express themselves into phenotype. At every step of the way environmental factors can intervene, join or overrule. Although at times geneticists have described the process as deterministically proceeding (and have used expressions such as “programming”) it has long been clear that that is an incorrect description. At times genes dominate a particular process; at others the environment.
Single-cause theories offer explanations for certain traits that are found exclusively in genes, “the environment”, child abuse, poverty, toxins or radiation, events happening in early childhood, prenatally, or parenting style. These theories are countered by the objection that multiple causes are the most common basis for human (and mammalian) symptoms and disorders. The principal reason that researchers attempt to establish causation is to use it to find pointers to cure, alleviate or prevent, disorders, symptoms or problems from occurring. In that respect it is useful to review and think through some examples that show that moving from known causation to remedy is often difficult:
- Phenylketonuria (PKU) is an example of a 100% genetic cause that can be matched by a wholly environmental (nutritional) means of prevention.
- After decades of comparing, there is no way of knowing whether the best remedy for mental disorders is psychoactive drugs, talking therapy or a combination of both.
- In established cases of clearly physical diseases such as terminal cancer or the neurocognitive disorders (dementias) there may be good evidence that forms of talking therapy would benefit the patients.
- For two of the most studied disorders with a suspected causal mixture of strong (polygenic) genetic causation combined with environmental influences, schizophrenia and autism spectrum disorder, there is no clear indication what the most effective treatment or combination of treatments should be, or how prevention or early diagnosis could be used.
Major battles have been fought about the question to what extent genetic and environmental (from conception) influences play the bigger role. Asserting that both always play some role is easy, but does not solve the problem of which role is stronger, nor address the complication of multiple causes including environmental, epigenetic and genetic causes. In addition, the influences are almost always probabilistic and don’t determine a particular outcome with certainty, and there is the difficulty of assessing indirect factors. Finally, there are influences early in life, whether genetic or environmental, that only impact in later life. This is true of genetic influences as much as of negative environmental experiences such as child abuse or disorganised attachment.
gives an overview of the main controversies. He writes especially about the shortcomings, hype and overpromises of geneticists in exaggerating what the breakthroughs in DNA and genome knowledge and in technology were going to allow. But it must be said that it is as possible to exaggerate the importance and dominance of epigenetic effects and of environmental influences as it is to exaggerate the role of genes. It is as extreme to believe in child sexual abuse as a single-cause explanation of mental disorder as it is to believe mental disorders are caused completely by genetic anomalies. An overview book about behavioural genetics is .
The most likely explanation for most severe problems and disorders is that there are contributions, not all of them equal, from the whole range of causes:
- genes, in the form of small contributions from multiple genes leading to an increased vulnerability
- environmental influences between conception and birth
- environmental influences around and shortly after birth
- attachment security in the first 1.5 years of life
- attachment issues during development
- lifelong influences of a person’s relational experiences and personal development, including natural or human-made trauma
- child abuse
- group, community, social and cultural influences
- use of drugs and substances
- influences from nutrition, nutritional deficiencies and toxicity
- more generally ecological influences
- learning in the sense of learning theory, including behaviourism
For further reading on genes and the environment in the context of human behaviour see. offers similar information but is more purely focused on behavioural genetics and research results than , who gives a wider overview. For an understanding of the more detailed issues of genetic influence and its role in evolution, two modern texts are and . They complement each other in that the former gives a more detailed picture of mechanisms and history, whereas the latter is up to date, modern, and deals with a wider range of the scientific issues surrounding the topic. is a shorter and perhaps more readable book that illustrates the issues with examples.
and (the latter being an updated version of a seminal text published in 1985) offer the most fully organised and compelling vision for what could replace the current neo-Darwinian and genetic mainline view but is so ambitious that the project has not been progressed much so far.
reports on one very large project to look at combined genetic and environmental causes for mental disorders in a systematic way, using modern methods for deriving causality from observations. It is impressive in its scope and methodology. is a short article in which he sets out a number of his conclusions about genetic causes for psychiatric disorders.
is an introductory textbook about human genetics for those who want to have a full overview and follow the details of the technical arguments for themselves.
deals with the public, political and ethical issues around behavioural genetics. is a classic that warns vigorously for the dangers of an overly gene-centred biology profession, psychiatry profession and society.
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Jablonka, E. & Lamb, M.J. (1995). Epigenetic Inheritance and Evolution: The Lamarckian Dimension.Oxford: Oxford University Press.
Kendler, K.S. (2006). “A gene for…”: the nature of gene action in psychiatric disorders. FOCUS, 4: 391-400.
Kendler, K.S. & Prescott, C.A. (2006). Genes, Environment, and Psychopathology: Understanding the Causes of Psychiatric and Substance Abuse Disorders. New York: Guilford Press.
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